What is desired from the honeycombs is firstly a large geometric surface area to accommodate the coating and secondly a low heat capacity in order that the honeycomb warms up rapidly to the operating temperature of the catalyst. In addition, a sufficient mechanical stability is required from the honeycombs in order to withstand the mechanical stresses through the pulsating gas flow and the vibrations of engine and vehicle. Moreover, the material of the honeycomb has to be resistant to the corrosive exhaust gas atmosphere at high temperatures.
The catalytic coating of a finished catalyst consists of finely divided thermally stable metal oxides on whose surface are deposited catalytically active platinum group metals. Suitable metal oxides are, for example, aluminum oxide, titanium oxide, silicon oxide, cerium oxide, zirconium oxide, zeolites and mixtures or mixed oxides thereof, and stabilizers such as lanthanum oxide and barium oxide. To apply these materials to the honeycomb, the pulverulent materials are, for example, suspended in water. Subsequently, this coating suspension is deposited onto the honeycombs by known processes, dried and consolidated by calcining.
The honeycombs may be of different structure. Even at a very early stage, spiral-wound honeycombs were used. They consist of one smooth and one corrugated sheet layer which are placed one on top of another and then wound up in a spiral and inserted into an outer tube. The two sheet layers form channels through which the exhaust gas can flow and enter into intensive contact with the catalytic coating.
In another design, the honeycomb is formed from a multitude of smooth and corrugated sheet layers, or differently corrugated sheet layers, arranged in alternation, in which case the sheet layers first form one or more stacks which are intertwined with one another. The ends of the sheet layers are on the outside and can be bonded with an outer tube. This forms many connections between sheet layers and outer tube, which increase the mechanical strength of the honeycomb.
The material used for the sheet layers are preferably aluminum-containing steel alloys, which are marketed, for example, under the trade name FeCrAlloy®. This is an iron-chromium-aluminum alloy. The thickness of the sheet layers is usually between 20 and 80 μm, preferably 50 μm.
It has been known for some time that the sheet layers can be provided with perforations and corrugations in order to influence the flow within the channels and/or to achieve transverse mixing between the individual flow channels. It is likewise known that slotted sheets can be used for the construction of metallic honeycombs. U.S. Pat. No. 5,599,509 proposes, for example, equipping the sheets with slots transverse to the flow direction of the exhaust gas in the entrance region of the honeycomb in order to reduce the heat capacity of the honeycomb in this region in a controlled manner.
As a result of the reduced thermal conductivity and heat capacity in the front part, the catalyst coating applied reaches its lightoff temperature more rapidly and hence also achieves a better conversion, for example for carbon monoxide and hydrocarbons. However, the improvement in the conversion depends significantly upon the percentage slot area of the sheet layers. The conversion is improved only within a range of slot areas between 20 and 50%. Above 50%, the hydrocarbon conversion decreases again because the catalyst material available decreases with increasing slot area and equal thickness of the catalyst layer. The improvements in the conversion are therefore observed only in the warm up phase. After the warmup phase, the reduced amount of catalyst material leads to a worsened conversion of the harmful substances in the exhaust gas. Moreover, such a catalyst, owing to the small amount of catalyst material, has a low aging resistance. Furthermore, the perforation in the sheet layers increases the back pressure of the catalyst, since turbulence forms at the holes. If the intention were to apply the same mass of catalyst material as in the case of an unperforated support with the same geometry to such a catalyst support, the layer thickness of the catalyst layer and hence likewise the back pressure of the catalyst would inevitably increase.
DE 103 14 085 A1 likewise describes metallic honeycombs composed of at least partly perforated sheet layers. The aim of the perforations is to enable transverse mixing of the exhaust gas streams of different flow channels. In the coating, it is therefore ensured that the holes are not closed by the coating suspension. For this reason, the honeycombs are coated in a vibration unit which generates a relative motion between the coating suspension and the support body and hence prevents closure of the holes.
U.S. Pat. No. 5,821,194 describes a metallic honeycomb composed of smooth and corrugated sheet layers. To improve the adhesion of a pre-coating material, the sheet layers are equipped with a multitude of holes, into which the pre-coating composition penetrates in the course of coating and forms a mechanical anchor between sheet layers and coating material. For this purpose, the diameter of the holes is about half the size of the thickness of the sheet layers and is thus from 25 to 35 μm. The holes are arranged at a distance of about 1 mm from one another.